Constant displacement stopping control

ABSTRACT

Stopping of a d-c servo motor is effected in two phases, during the first of which the velocity is brought to half its initial value after initiation by a stop signal. A reference single shot and a proportional single shot determine the timing of the second phase, these circuits acting together to generate a stopping waveform which traverses a constant displacement, independent of motor deceleration rate.

United States Patent 1151 3,668,494 Agin 1451 June 6, 1972 [54] CONSTANT DISPLACENIENT 3,500,163 3/1970 Moritz ..3l8/373 STOPPING CONTROL 3,541,418 11/1970 Agin et al. ..318/6l2 2,821,672 1/1958 Sichling et al... 187/29 X [72] Imam" 2,846,026 9/1958 Gott et a] ..187/29 [73] Assignee: International Busines Machines Corpora- 3,240,290 3/1966 Pohlman ..187/29 tion, Armonk, N.Y. Primary Examiner-T. E. Lynch 7 [22] Filed June l9 Attorney-Hanifin and Jancin and Francis V. Giolma [21] Appi. No.: 48,249

[57] ABSTRACT [52] U.S. Cl ..318/373, 318/612, 318/561 stopping f a 1. Servo m;- i ff t d in two phases, during [51 Int. Cl. .1102]! 3/10 the fi f which the velocity is brought to half its initial value [58] FIG! of Search "3 1 8/373, 387, 390, 399, 447, afier initiation by a stop signal A reference single shot and a 318/6; 187/29 proportional single shot determine the timing of the second [56] References Cited phase, these c1rcu1ts acting together to generate a stoppmg waveform which traverses a constant displacement, indepen- UNITED STATES PATENTS dent of motor deceleration rate. 54,730 10/1964 Houldin et a1. ..318/302 3 Claims, 12 Drawing figures RFFERENFE scavo 111011 (1100111101 5101*) I 58 i2 I i sec 6 0 1 '3 OR I DRIVER 5m? 1 101011101111 111011 22 5mm $1101 HALF a PS8 24 2s iiiiiiii OR DECELERATION VELOCITY ERROR DISPLACEMET FIXED TIMING I FIG. 2a

- ZERO VELOCITY TACHOMETER CONTROL FIG. 2b FIG. 2C

,18 REFERNCE SERVO LATCH L (00 ORNOT STOP) I 33 \20 FIG. 3

12 14 1 u X #88616 flo IDRIVERI (p I 35 W STOP PROPORTIONAL do LATCH 22 SINGLE 5H0T 26 HALF IEIIRBI &/ OR

NR6. 8 t2 lNVE/VTOR. GERALD J. AGIN HQ 5 M 21% A TTORNE Y PATENTEUJUR 6 I972 3668,49 1

SHEET 2 OF 3 SERVO LATCH (NOT STOP) REFERENCE SINGLE SHOT PSS INPUT L PSS OUTPUT T h I STOP LATCH HALF VELOCITY DETECTOR I; T

MOTOR VELOCITY 36 OSC SS 38 n OR DRIVER ,10 HALF 8 TEEERR STOP J LATCH I3 ,40 --s N FIRST 7 PHASE RECOVERY 25A LATCH 42 28 #586 R F 38 4 SERVO LATCH OR I 88 (NOT STOP-G0) 29 DLY.

' PROPORTIONAL ,18 REFERENCE 22 a Pss I 38 -2 SINGLE SHOT FIG. 6

PATENTEDJUH 6 I972 NOT STOP REFERENCE SS PSS INPUT PSS OUTPUT OR 26A RECOVERY SS FIRST PHASE LATCH SET STOP LATCH STOP LATCH STOP INTECRATOR MOTOR DRIVE VELOCITY NOT STOP REFERENCE ss PSSINP PSS 0 0R 26A ERY ss PHASE LATCH SET STOP LATCH STOP LAT STOP m1 0R MOTOR DRIVE VELOCITY NOT STOP ENCE SS INPUT PSS OUTPUT OR 26A RECOVERY'SS FIRST PHASE LATCH SET STOP LATCH STOP LATCH STOP INTEGRATOR MOTOR DRIVE VELOCITY SHEET 3 OF 3 VERY HOT I I 1 I IlIIII|IIIIIIIIIIIIIIIIIIIIIIIIIIITI' FIG. 7c

1 CONSTANT DISPLACEMENT STOPPING CONTROL FIELD or INVENTION This invention relates to d-c servo motors and it has reference in particular to a constant displacement stopping control for a d-c printed circuit motor driving a carriage for a printer.

PRIOR ART I-Ieretofore, incrementing of d-c servo motors has been effected by accelerating the motor quickly to a given velocity, maintaining this velocity by servo control, and then decelerating the motor quickly to a stop. The deceleration is usually initiated by a signal from a displacement transducer (sometimes called a precoincidence signal), the length of the deceleration pulse being enough to bring the motors velocity to zero, this being determined by a fixed timing source such as a single shot, by a tachometer, or by a stop integrator such as disclosed in the IBM Technical Disclosure Bulletin for May 1969, on page 1,697.

SUMMARY OF THE INVENTION Generally stated, it is an object of this invention to provide a new and novel control circuit for a d-c servo motor.

More specifically, it is an object of this invention to provide for using a reference single shot and a proportional single shot to determine the final phase of a two-phase stopping sequence for a d-c servo motor.

Another object of the invention is to provide for using a velocity detector which fires twice during stopping to reset a stop latch during the two phases of a stopping sequence fora d-c servo motor.

An important object of the invention is to provide for using a reference single shot and a proportional single shot to set a stop latch during the second phase of a two-phase stopping sequence for a d-c servo motor.

Yet another object of the invention is to provide for using a half-velocity detector for resetting a stop latch during a d-c servo motor stopping sequence.

It is also another important object of the invention to provide for using a reference single shot and a proportional single shot to establish a coasting interval in the middle of a deceleration waveform to provide constant stopping displacement of a d-c servo motor.

The foregoing and other objects, features, and advantages of the invention will be apparent from the following, more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawing.

DESCRIPTION OF THE DRAWING in one of its forms.

FIG. 4 shows a plurality of timing curves and a typical motor velocity waveform illustrating the operation of the stop control of FIG. 3.

FIG. 5 shows curves illustrating the stopping velocity waveform of the stop control circuit of FIG. 3 as a function of temperature.

FIG. 6 is a more detailed schematic block diagram of a constant displacement stop control system for a d-c servo motor embodying the invention.

FIGS. 7a through 7c show timing curves illustrating the operation of the stop control system of FIG. 6 under difierent temperature conditions.

FIG. 8 illustrates a velocity waveform used in connection with the derivation of an optimum proportional single shot ratio covered in the appendix.

DESCRIPTION OF PRIOR ART A common method of incrementing d-c servo motors is to accelerate the motor quickly to a given velocity, maintain this velocity by servo control, then to decelerate quickly to a stop. A typical velo'city waveform is shown in FIG. 1. The deceleration is usually initiated by a signal from a displacement transducer (sometimes called a precoincidence signal). The length of the deceleration pulse should be enough to bring the motor's velocity exactly to zero, and may be controlled by a fixed timing source (single shot), by a tachometer, or by a stop integrator such as described in the IBM Technical Disclosure Bulletin for May 1969, on page 1,697.

If the motors terminal resistance varies, due for instance to a temperature rise, the rate of deceleration will vary, and an adjustment must be made in the length of the stopping pulse. A fixed timing source will not compensate for the change in deceleration, but a tachometer will terminate the reverse drive when the velocity is zero. The stop integrator will maintain a fixed change in velocity (Aw) over varying deceleration. These stopping waveforms are shown in FIG. 2.

Even though the tachometer and stop integrator will bring the motor to zero velocity, the angular displacement traversed by the motor from the initiation of deceleration to the point where the motor stops, is still a function of the deceleration rate. The difference in displacement is indicated in FIG. 2 by the shaded areas labeled (A0). In many practical-systems, this variation is undesirable. It is the object of the control system described below to eliminate this dependence of final position on the deceleration rate.

DESCRIPTION OF A PREFERRED EMBODIMENT The system of the present invention makes use of the pro portional single shot of U. S. Pat. No. 3,541,418, which issued Nov. 17, 1970, to Gerald J. Agin et al, entitled Proportional Damping for Motor Drive," application Ser. No. 665,5 39, filed Sept. 5, 1967. Its operation may be summarized as follows: An input pulse charges a capacitor to a level determined by the length of the pulse. During the discharge of the capacitor the output is switched; the length of the output pulse is determined by the level the capacitor was charged to by the input pulse. The ratio of output pulse length to input pulse length is equal to the ratio between'the discharging and charging time constants of the circuit.

A simplified logic circuit diagram of a constant displacement deceleration system embodying the invention is shown in FIG. 3 and its waveforms in FIG. 4. As shown, a Stop latch 10 is provided to control the Driver 11 for a Servo Motor 13 in order to supply; reverse drive thereto for stopping. The Stop latch 10 is set in response to termination of a Go or Not Stop signal from a servo latch (not shown), which is applied to set the latch through an OR 12, Inverter 14, and Single Shot 16. The same servo latch signal is applied to an Inverter l8 and a Reference Single Shot 20 to gate the Off output of the Stop latch 10 in AND 22 for controlling a Proportional Single Shot 24, to provide a signal through OR 26 back to OR 12 to again set the Stop latch 10. The Stop latch 10 is reset by applying the On output of the latch 10 through a Half-Velocity Detector 30 such as, for example, the current integrator described in the IBM Technical Disclosure Bulletin for May I969 on page 1,697, with the charging rate doubled.

In place of the usual zero velocity detector, which fires only once, the present circuit utilizes the Half-velocity Detector 30, which fires twice. The Detector 30 is connected to a Resistor 31 in the armature circuit of the Motor 13 to respond to a predetermined integral of deceleration (change in motor velocity), such as, for example, half the normal velocity of the motor. The first phase of the stopping sequence is initiated by the termination of the Not Stop or Go signal from the servo latch, which provides an output from the Inverter 18 to start the end of the Reference single shot pulse. The Proportional single shot capacitor is charged during the interval between the end of the first stopping phase and the timeout of the Reference Single Shot 20. If the ratio of its time constants is unity, the second stopping phase will begin at an equal interval after the timeout of the Reference Single Shot 20.

The result of this action is to introduce a coasting interval in the middle of the stopping sequence. If the motor is warm and its deceleration low, it will take longer to reach half velocity and travel farther to reach it than when the motor is cold and its deceleration high. However, the additional coasting displacement at half velocity, when the motor is cold, should exactly make up forthe difference in displacement.

FIG. shows the constancy of displacement in a different way. Two velocity wavefomis are shown: one for hot and one for cold operation. The difierence in displacement is the area between the two curves. But this area is the sum of the two shaded areas of the figure, one a positive area (with hot wavefonn above) and one for a negative area (with the cold waveform above). Since the two shaded areas are equal, their sum will be zero, and the displacement on stopping will not be afunction of deceleration rate.

In some cases it is desirable to achieve a reduced level of drive in the second phase of deceleration, to reduce the jerk" orfirst derivative of acceleration and to minimize the resulting mechanical vibrations of the system. In this case the ratio of the time constants of the Proportional single shot will have to be adjusted to other than unity. The optimum ratio is (l aI/a2 )/2 where a1 is the deceleration rate during the first phase of stopping, and a2 is the deceleration rate during the second phase. For a derivation of the optimum rate refer to the Appendix.

The logic circuit of FIG. 3 is partially incomplete in that no provision is made for the case where the first phase of deceleration approaches the length of the reference single shot.

FIG. 6 shows the complete control system necessary to implement constant displacement stopping, and FIG. 7 shows its waveforms under various operating conditions. The additional features over what is shown in FIG. 3 include I an Oscillator 32 for applying pulses through a Duty-Cycle Single Shot 34, OR 36, and AND 38 to the motor Driver 11 to give a variable rate of deceleration (dependent on the duty cycle), (2) a First Phase latch 40, which is connected to be set by the Not Stop or Go signal from the servo latch to override a pulsed drive from Oscillator 32 and provide a dc level signal through the AND 38 in conjunction with the On output of the Stop latch 10, (3) a Recovery Single Shot 42, which connects the Off output of the First Phase latch 40 through OR 26A to control set of the Stop latch through Inverter 28 and Single Shot 29 to guarantee the Stop Integrator 30 sufficient time to discharge for the next cycle, (4) the First Phase latch 40 and the Recovery Single Shot 42 together guarantee that if the first phase of stopping overruns the Reference Single Shot 20 signal and the Proportional Single Shot 24 never fires, the second phase will still be initiated, (5) a short delay by means of Delay 50 to eliminate a transient or spike on the output of the OR 26A when the Proportional Single Shot 24 output and input are switching simultaneously.

A useful byproduct of this method of stopping is that the final stopping position may be adjusted with respect to the position of the precoincidence signal by varying the Reference Single Shot 20 instead of mechanically moving a displacement transducer, which provides the precoincidence signal as was done heretofore.

In operation, a termination of the Not Stop or Go signal from the servo latch turns on the Reference Single Shot 20 through the Inverter 18. The Stop latch I0 is set through the OR 26A, Inverter 28, and the Single Shot 29 so as to gate an output from AND 38.

When the Stop latch I0 is first set, the First Phase latch 40 will still be on, and the pulse drive from the Oscillator 32 will be overridden by the d -c level output of the First Phase latch 40 through OR 36 to maintain a solid drive through AND 38 to the Driver 1 l for decelerating the Motor 13. The Stop latch 10 and the First Phase latch 40 are reset by the Stop Integrator 30 to terminate the first phase of the stopping sequence. The Proportional Single Shot 24 times out and through OR 26A, Inverter 48, and Single Shot 50, again sets the Stop latch 10. This time the output of the Stop latch 10 gates the pulse output of the Oscillator 32 through the Duty-Cycle Single Shot 34, OR 36, and AND 38 to provide a pulsed drive to the driver during the second phase of the stopping sequence. The Stop latch 10 is again reset by the Stop Integrator 30 .to terminate the sequence.

From the above description and the accompanying drawing it will be seen that there is provided a combination of a reference single shot and a proportional single shot to establish a coasting interval in the middle of a deceleration waveform for a d-c servo motor such that the duration of the coasting interval compensates for changes in stopping displacement that would otherwise result from a variation in deceleration rate. A control circuit embodying the invention is capable of extremely accurate positioning and is substantially independent of temperature effects. While the Stop Integrator has been described as a Half-velocity Detector, other predetermined proportions may be used as desired.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

APPENDIX DERIVATION OF OPTIMUM PROPORTIONAL SINGLE SHOT RATIO Referring to the velocity waveform in FIG. 8 where 2 is a constant At=T-t =T-1/2 i a1 To find the stop displacement (area under the curve):

1. In a stop control for a d-c motor having an armature energized by driver circuit means,

bistable switch means connected to said driver circuit means operable to turn on said driver circuit means to provide reverse energization of said motor armature for stopping said motor,

circuit means connected to said bistable switch means for effecting operation thereof a first time in response to a stop signal to start a first phase of a stop sequence,

integrating means connected to said bistable switch means and said driver circuit means responsive to the value of 5 armature current of said motor to render said bistable switch means inoperative and terminate said first phase to provide a first stop phase having a duration determined by the value of the armature current of said motor during said first phase, and

additional circuit means connected to said bistable switch means to render said bistable switch means operable a second time to start a second phase in the stop sequence, said integrating means operating to again render said bistable switch means inoperative and terminate said second phase and provide a second phase having a duration detem'iined by the value of the armature current during said second phase.

2. The invention as defined in claim 1 characterized by said bistable switch means comprising a stop latch having set and reset operating conditions, and said additional circuit means comprising a reference single shot connected to respond to said stop signal and a proportional single shot connected to said reference single shot and to said stop latch to be activated by said stop latch being reset.

3. The invention as defined in claim 2 characterized by said reference single shot being connected to gate means with the off output of said stop latch to activate said proportional single shot to determine the duration of the inoperative time of said stop latch. 

1. In a stop control for a d-c motor having an armature energized by driver circuit means, bistable switch means connected to said driver circuit means operable to turn on said driver circuit means to provide reverse energization of said motor armature for stopping said motor, circuit means connected to said bistable switch means for effecting operation thereof a first time in response to a stop signal to start a first phase of a stop sequence, integrating means connected to said bistable switch means and said driver circuit means responsive to the value of armature current of said motor to render said bistable switch means inoperative and terminate said first phase to provide a first stop phase having a duration determined by the value of the armature current of said moTor during said first phase, and additional circuit means connected to said bistable switch means to render said bistable switch means operable a second time to start a second phase in the stop sequence, said integrating means operating to again render said bistable switch means inoperative and terminate said second phase and provide a second phase having a duration determined by the value of the armature current during said second phase.
 2. The invention as defined in claim 1 characterized by said bistable switch means comprising a stop latch having set and reset operating conditions, and said additional circuit means comprising a reference single shot connected to respond to said stop signal and a proportional single shot connected to said reference single shot and to said stop latch to be activated by said stop latch being reset.
 3. The invention as defined in claim 2 characterized by said reference single shot being connected to gate means with the off output of said stop latch to activate said proportional single shot to determine the duration of the inoperative time of said stop latch. 